Maintaining Area or Room Pressurization in Manufacturing and Healthcare
Finding and maintaining the optimal level of pressurization is a difficult balancing act. Too large of a differential and you will have problems with temperature and humidity control; perhaps also, complaints from people using the space, but when the differential is too small, processes, products, and people could be at risk.
The key to effective area pressure control is a sensitive and reliable room pressure monitor coupled with a highly accurate pressure transmitter. This equipment is best specified and installed when the room is first constructed or undergoing refurbishment; however, solutions are available for retrofitting to existing area pressurization systems. Here is what you need to know.
Positive and Negative Pressurization
Start by considering what pressurization is for. The goal is to either exclude particulates from a room, or alternatively, to keep them within a defined space or enclosure, which can be permanent or temporary.
Exclusion is achieved by maintaining pressure in the room above that in surrounding areas – a positive pressure differential. Air flowing out, from high pressure to low, prevents entry of air that contains particulates. Inclusion or retention is achieved by creating a negative pressure differential. In this scenario each time a door is opened air flows into the room, preventing particulates from leaving.
Positive Pressurization Applications
Clean rooms, as used in semiconductor manufacturing, high precision metrology and surgical environments employ positive pressurization. Typically, to ensure cleanliness, incoming air is first passed through a high efficiency filtration system.
Negative Pressurization Applications
In other situations, it is important to prevent contaminants from leaving the room. One example would be in pharma manufacturing processes that generate or release dust.
Another application is for an area, temporary or permanent, dedicated to testing and/or treatment of COVID-19 patients. It is critical to patients and staff in adjacent areas to avoid exposure to airborne pathogens like the coronavirus or other viruses and diseases that may be transmitted through contaminated air from quarantined areas.
An ASHE note on room pressurization sorts medical environments into positive and negative applications. COVID-specific recommendations are detailed under, “Negative Pressure Patient Room Options.”
Air Quality Control and its Impact on Area Pressurization
When considering applications like pharmaceutical manufacturing, it raises the importance of air quality, as well. First, humidity control impacts the manufacture of medicines. Many powder and coating processes are sensitive to moisture in the air. This shows that pressurization control requires parallel management of temperature and humidity.
Optimizing the Pressure Differential
How much difference is needed for safety and cleanliness in these situations? The answer is, not much. In general, a differential of 0.02 to 0.05 in wc (inches of water column) is enough to create the required airflow. For clean rooms, the former US Federal Standard 209E, (superseded by ISO 14644,) discussed a need for 20 ACH (air changes per hour) and a differential of at least 0.05 in wc.
Higher levels can create drafts that annoy room occupants and make it harder to open or close doors. Of greater concern, are the impacts on temperature and humidity control.
A higher differential correlates with more air changes per hour. That means a larger volume of air to heat or cool. Rooms that need a pressure differential, especially one that is positive, usually also require temperature and humidity to stay within close limits. A negative pressure pharma production area will likely also have strict humidity requirements.
Thus, the result of an excessive pressure differential is that either the airflow management system works harder to maintain those limits, or the room deviates outside them (which could have very costly consequences.) The goal then is that pressure differentials be kept as small as the control system can achieve.
Pressure Differential Measurement Technology
There are two possible approaches to measuring pressure differential. Either find the difference in static pressure inside and outside the controlled environment or measure rates of airflow in and out and extrapolate the difference.
In practice, many modern systems employ a cascading combination of the two techniques. For example, it might be appropriate to track airflow but use differential pressure monitoring to set an alarm. This helps in keeping room temperature within narrow limits. Alternatively, it could be a change in airflow that triggers an alert while differential pressure monitoring controls the air change rate.
Choosing the most appropriate method depends on application-specific details. Factors such as room size and construction, frequency of entrance/exit, and the size and arrangement of the ducting all have an impact. An airflow monitoring specialist should be asked to make a recommendation after reviewing the requirements and the proposed installation.
From the preceding discussion it should be clear that measurement accuracy is a key consideration. The ideal room pressure monitor will have fine measurement resolution, rapid sensor response, and a high sampling or measurement rate. While some users may be satisfied with an alarm when pressure goes outside set limits, most will want the room pressure monitor to interface with the airflow control system for airflow management.
For more precise pressure control, a “smart” pressure transmitter such as the VELTRON II from Air Monitor may be required. This highly accurate unit handles ultra-low differential pressures and is intended for the most demanding applications.
A feature of the VELTRON II pressure transmitter of interest is its ability to essentially “self-calibrate.” This refers to an automatic zeroing circuit that eliminates drift of the output signal drift due to thermal, electronic, or mechanical effects. It also avoids the need for initial or periodic transmitter zeroing.
Installation and Retrofit Considerations
In general, precise airflow measurements need long, straight duct runs. A feature of Air Monitor airflow measurement stations is that they provide higher accuracy over shorter lengths of straight duct than competing products. This is achieved by using multi-point averaging pitot tube technology coupled with ultra-low, highly accurate, differential pressure measurement.
Pitot technology enables measurement of true velocity pressure, (total pressure – true static pressure). Other technologies measure a reference pressure on the back or side of the probe which result in a measurement that is not true velocity pressure as derived from Bernoulli’s Equation.
Another benefit of Air Monitor equipment is the ease with which it can be applied to existing systems. Furthermore, this is feasible with minimal disruption and downtime.
Accurate and Reliable Differential Pressure Measurement Technology
The key to precise pressure differential management is accurate and reliable measurement technology. With this it is feasible to minimize air changes and associated heating, cooling, and humidification requirements while still maintaining safety and product quality as needed. To find out more about how Air Monitor can help with your area pressurization requirements, request a quote from one of our application specialists today!